CoreData and "conflictList" error during save

FROM : Dex Morgan
DATE : Mon May 12 19:18:30 2008

Hello,
I'm using core-data in a program of mine. It uses two context where 
one of these is used in a second thread to perform saving operation.
Sometimes at the end of all operations when I perform a save operation 
on my main context I receive a
"conflictList" error.
Is there any way to avoid it by merging changes?
I've tried with
[[self managedObjectContext] processPendingChanges];
[[self managedObjectContext] commitEditing];

before save: but nothing is changed.
TIA